1. Field of Invention
This invention relates to the field of data communication and, in particular, the field of RFID data communication.
2. Description of Related Art
RFID is an acronym for radio frequency identification. RFID is a term used for a variety of communication systems using radio frequencies from about 100 kHz to over 6 GHz. RFID systems are characterized by relatively simple devices (often referred to as tags, transponders, tokens etc.) deployed in large numbers that communicate digital information with relatively fewer and more complex devices (often called readers, interrogators, scanners, etc.). Tags are affixed to objects or places to be identified, monitored and tracked. Readers obtain digital information from tags and often send digital information to tags for storage and control. Readers interface to external data systems such as host computers. Data transfer between tags and readers can be one-way (read only tags) or two-way (read write tags). Radio technologies include inductive (or sometimes capacitive) coupling at low frequencies. Readers using propagating electromagnetic waves send data to tags by modulation of transmitted radio signals. Since the tags are usually relatively simple, various forms of amplitude modulation are used since amplitude modulation is relatively simple for the tag to recover using direct detection techniques with a diode rectifier or equivalent. Tags can send data to readers using modulated backscatter or active radio transmission. Tags can use batteries for power or alternatively, use energy harvesting techniques such as rectification of RF signals, conversion of visible or infra red light using photocells and the like.
RFID systems have become popular for applications in item management, warehousing, logistics, the supply chain, factory automation, electronic toll collection, wireless access control, security, wireless passes for ski lifts, busses and subways, tracking rail cars, intermodal containers, trucks, identification of animals and many other such applications.
A given RFID system uses one of a variety of communication protocols, some proprietary and others based on ad hoc or formal national and international standards. RFID systems are widely deployed. As applications have grown and capabilities have increased, compatibility between systems and with system upgrades has become a problem. It is common to attach several tags, each using a different protocol, to an object to enable that object to interface with RFID readers of different protocols. One example is for interstate trucks that routinely pass through ports of entry, use toll roads and cross bridges each using an RFID system of a differing protocol. It is also common for reader stations to use several readers of different protocols, or single readers implementing several protocols to be able to interface with tags of different protocols. Such systems may experience a reduced throughput due to the necessity of processing differing protocols in series.
Tags using differing protocols can overlap service areas due to expansion of neighboring systems using products produced to different protocols. Differing protocols can also exist in a region due to upgrades of capability and thus new or expanded capability protocols. The use of multiple protocol tags and multiple protocol readers can ease the transition to new systems without requiring a mass recall and replacement of tags, which is a time consuming and expensive process.
As usage increases, it is often desirable to expand the capabilities of an RFID system that is already in place. Enhancements and updates may include faster data rates, changing the coding, adding command codes, increasing memory size, adding authentication and/or encryption or modifying other such details of the communication protocol. To make use of an updated tag, an updated reader is required. If a complete tag replacement is to be avoided due to concerns of cost and service disruption, the capability to read existing tags must be retained. Thus, the updated reader must manage two populations of tags. Likewise, if the tags using a new protocol are to be introduced before readers using the new protocol, the new tags must also respond to old readers. Tags, readers, communications protocols and methods of use must be carefully designed to minimize interference and maintain the desired throughput.
Some companies have done well implementing multiple protocols within a system. For example, TransCore™ RFID systems for electronic toll collection include protocols from ISO 10374, ATA, AAR, ISO 18000-6B, Title 21 of the state of California, and extensions of these standards. These protocols include read-only as well as read-write tags. Some installations have started with the ISO10374/ATA/AAR protocol, migrated to Title 21, and then to ISO 18000-6b. These protocols are quite different from each other and careful equipment design, manufacture, configuration and installation are needed for successful implementation including enforcement (catching vehicles evading paying the toll). Performance requirements of an electronic toll collection system are demanding. The available time and frequency spectrum must be carefully managed and used by the RFID system to achieve the high reliability demanded by toll operations while remaining within radio regulations. Adding an additional service, such as authentication of tags to increase security and reduce fraud, requires additional data capacity. The system design of a given toll collection system using several RFID protocols may not be able to increase data capacity through the conventional methods of increases in data rates, using additional time per transaction, or other straightforward methods used by more expensive and power consuming wireless systems such as higher orders of modulation (such as 16 QAM or 64 QAM) of digital radio signals used in WiFi systems (radios complying to the standard IEEE 802.11).
Thus, there is a need for a way to increase channel data capacity in the uplink, downlink or both in an RFID system in such a way that normal operations are not affected when upgraded equipment is used in an existing system with legacy equipment. An updated tag is needed that can signal its ability to utilize an increased channel data capacity without disrupting normal communications with a non-upgraded reader, and an upgraded reader is needed that can communicate with non-upgraded as well as upgraded tags without disrupting normal communications. It is also desirable that use of the upgraded channel data capacity does not change the timing or bandwidth requirements of the system. The increased capabilities of the tags and readers are required to be economical and not increase power consumption or introduce other undesirable characteristics that would impede the growing widespread use of RFID systems.
As technology evolves, there is an increasing pressure for improvements. Increased security, improvements in channel data capacity, implementation of additional functions such as sensor inputs and the like are desired. However, establishing a new, or green field, application is difficult. Often, improvement is incremental and must be backward compatible with older equipment.
Increased security may use techniques of authentication of tags and/or readers. The authentication of tags prevents use of fraudulent, cloned, or otherwise non-genuine tags. The authentication of readers by tags prevents unauthorized access to tag functions and data. Authentication is becoming an increasing need for RFID systems as these systems become widespread. The authentication process involves encryption techniques and requires additional data to be transmitted between tag and reader. Many presently installed RFID systems have limits on the amount of data transmitted. If tags transmit additional data, upgraded readers can be designed to respond, but older readers not designed to handle additional data may fail and become confused with the additional data resulting in failures of the system to operate properly. For example, to add authentication to an existing tag protocol, some part of the transponder current response could be used to add authentication data. This would require changing some of the data that the transponder currently sends to the reader. This change in data would cause the reader to see this transponder as a new type of transponder. Because of this, currently installed readers would treat the new transponders differently from the old non-authenticated transponders, and the overall transaction performed by the reader would not be completed.
U.S. Pat. No. 4,075,632 teaches a method of encoding a slowly changing parameter, such as temperature, in the response of the tag as a change in the frequency of the data clock. The digital data stored in the memory of the tag is sent by the tag to a reader using modulated backscatter. A reader processes the signal to determine the digital information carried by the amplitude modulated backscattered signal and to extract the clock frequency thus recovering the value of the slowly varying parameter. However, the '632 secondary information signal has insufficient data capacity to convey an adequate amount of data in a limited time to provide the function of tag authentication discussed in the previous paragraph.
All references cited herein are incorporated herein by reference in their entireties.